Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two common ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during processing with a developer or other processing solution. For positive-working printing plates, the exposed regions are dissolved in a processing solution and the unexposed regions become an image.
Various negative-working radiation compositions and imageable elements are described in U.S. Pat. No. 6,309,792 (Hauck et al.), U.S. Pat. No. 6,569,603 (Furukawa), U.S. Pat. No. 6,893,797 (Munnelly et al.), and U.S. Pat. No. 6,787,281 (Tao et al.), and in U.S. Patent Application Publications 2003/0118939 (West et al.), 2005/0008971 (Mitsumoto et al.), and 2005/0204943 (Makino et al.), and in EP Publications 1,079,276A (Lifka et al.), EP 1,182,033A (Fujimaki et al.), and EP 1,449,650A (Goto).
Various negative-working imageable elements have been designed for processing or development “on-press” using a fountain solution, lithographic printing ink, or both. For example, such elements are described in U.S. Patent Application Publication 2005-263021 (Mitsumoto et al.) and in U.S. Pat. No. 6,071,675 (Teng), U.S. Pat. No. 6,387,595 (Teng), U.S. Pat. No. 6,482,571 (Teng), U.S. Pat. No. 6,495,310 (Teng), U.S. Pat. No. 6,541,183 (Teng), U.S. Pat. No. 6,548,222 (Teng), U.S. Pat. No. 6,576,401 (Teng), U.S. Pat. No. 6,899,994 (Huang et al.), U.S. Pat. No. 6,902,866 (Teng), and U.S. Pat. No. 7,089,856 (Teng).
U.S. Patent Application Publications 2005/0170282 (Inno et al.), 2005/0233251 (Kakino et al.), 2003/0068575 (Yanaka), 2006/0046189 (Kunita et al.), and 2007/0072119 (Iwai et al.), and EP Publications 1,557,262 (Inno et al.), 1,588,858 (Kakino et al.), 1,614,541 (Callant et al.), 1,736,312 (Callant et al.), and 1,754,614 (Kakino et al.), 1,717,024 (Inno et al.) describe lithographic printing plate precursors that contain a discoloring agent or system capable of generating a color change upon exposure for providing print-out.
Publication SPCOM000170014D that can be read at IP.com describes a negative-working, on-press developable lithographic printing plate exhibiting good printout, good run length, and consistency in developability no matter the type of aluminum-containing substrate that is used.
Copending and commonly assigned U.S. Ser. No. 11/838,935 (filed Aug. 15, 2007 by Home, K. Ray, Knight, Huang, Tao, and Munnelly) describes the use of specific spirolactone or spirolactam leuco dye color formers in the imageable layer of negative-working imageable elements.
In addition, copending and commonly assigned U.S. Ser. No. 11/762,288 (filed Jun. 13, 2007 by Yu and K. Ray) describes on-press developable negative-working imageable elements that include a phosphate (meth)acrylate and do not require a post-exposure baking step.
Problem to be Solved
After imaging, printing plates may be inspected to make sure the desired image has been obtained. For printing plates normally processed (or developed) off-press, this inspection can occur easily before mounting on the printing press. The plate manufacturer often adds a colorant to the imaging composition to facilitate this inspection.
For imaged elements that are to be developed on-press, the image is not easily identified. Adding colorant to on-press developable imaging compositions compromises plate shelf life, on-press developability, or imaging sensitivity, and the colorant may color-contaminate printing press inks. The problem of development variability is particularly acute when the imageable layer is disposed on a sulfuric acid-anodized aluminum substrate.
Unfortunately, imageable elements that have good run length and improved developability may exhibit poorer printout (color contrast between imaging and non-imaged regions after exposure and before development) in the resulting image.
Thus, there is a need for adequate print-out that provides visibility to the image on the printing plate before on-press development. Further, there is a need to provide improved printout without any degradation in run length, developability, or other desired properties.